The rotary electrical machines which are produced in a large range of power levels and speeds are being used increasingly at present in the motor vehicle industry, as a result of considerations of protection of the environment.
They have applications in vehicles of the all electric type, as well as in low CO2 emission vehicles of the so-called mild-hybrid and full-hybrid types.
The mild-hybrid applications generally relate to electrical machines of approximately 8 to 10 kW, for example an electric motor which is fitted on the front surface of a thermal engine, and is coupled to the latter by means of a drive belt. With an electric motor of this type, it is possible to reduce the thermal motorisation capacity (engine downsizing), by providing electric torque assistance which supplies additional power, in particular during revving up. In addition, traction at low speed, for example in an urban environment, can also be ensured by this same electric motor.
Applications of the full-hybrid type generally relate to 30 to 50 kW motors for architectures of the series and/or parallel type with a more highly developed level of integration of the electric motor(s) in the traction chain of the vehicle.
These machines are polyphase machines which are supplied by the on-board battery by an inverter which makes it possible to generate the different phase currents.
The inverters which are used most commonly are inverters with two levels, i.e. they consist of a power bridge comprising a plurality of arms (generally one arm per phase), each comprising two power semiconductors which function in switching. A middle point of each pair is connected to a phase of the rotary electrical machine.
The power semiconductors are either bipolar transistors, in which case they are associated with free wheel diodes, or transistors of the MOSFET type, the intrinsic diode of which permits circulation of the current in two directions.
These semiconductor switches are controlled in a complementary manner by an electronic control unit, such as to generate the phase voltages necessary for functioning of the electrical machine according to a well-known technique of pulse width modulation (PWM).
The high power level reached by these machines is leading manufacturers to try to improve the performance of the associated inverters, by limiting the switching losses and the conduction losses.
In document FR2895597, it is proposed to limit the switching losses. For this purpose, a common mode voltage is adjusted such that one of the phase voltages is connected to earth or to the +BAT, with precedence given to whichever of these passes the stronger current. Thus, there is constantly an arm which does not switch, which makes it possible to reduce the losses in the inverter.
In the PWM technique, a switching frequency is generally fixed, whereas the wave forms of the phase voltages (sinusoidal for example) are obtained by modulating the duty cycles to an electrical frequency which depends on a speed of rotation of the machine.
In general, the electrical frequency is several hundred Hertz. The junction temperature of the MOSFETs (which reflects the losses) does not follow this instantaneous variation, since it is filtered by the thermal capacitors of the chip-case-substrate assembly.
However, in the phases of starting of the rotary electrical machine, or with the rotor blocked, the electrical frequency is very low, or zero. The thermal filtering of the assembly no longer operates: the losses to be taken into account are instantaneous losses, and not in the medium term.